A need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. A need also exists for a replacement for the conventional cathode-ray tube used in many display devices including computer monitors, conventional and high-definition television receivers and large-screen displays. Both of these needs can be satisfied by display devices that incorporate a light valve that uses as its light control element a spatial light modulator based on a ferroelectric liquid crystal (FLC) material.
FLC-based spatial light modulators are available in either a transmissive form or in a reflective form. The transmissive spatial light modulator is composed of a layer of a FLC material sandwiched between two transparent electrodes. The FLC material is preferably a surface-stabilized FLC material. One of the electrodes is segmented into an array of pixel electrodes to define the picture elements (pixels) of the transmissive spatial light modulator. The direction of an electric field applied between each pixel electrode and the other electrode determines whether or not the corresponding pixel of the transmissive spatial light modulator rotates the direction of polarization of light falling on the pixel. The transmissive spatial light modulator is constructed as a half-wave plate and rotates the direction of polarization through 90.degree. so that the polarized light transmitted by the pixels of the spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
Reflective spatial light modulators are similar in construction to transmissive spatial light modulators, but use reflective pixel electrodes and have the advantage that they do not require a transparent substrate. Accordingly, reflective spatial light modulators can be built on a silicon substrate that also accommodates the drive circuits that derive the drive signals for the pixel electrodes from the input video signal. A reflective light valve has the advantage that its pixel electrode drive circuits do not partially occlude the light modulated by the pixel. This enables a reflective light valve to have a greater light throughput than a similar-sized transmissive light valve and allows larger and more sophisticated drive circuits to be incorporated.
As with the transmissive spatial light modulators, the direction of an electric field (in this case between the transparent electrode and the reflective electrode) determined whether or not the corresponding pixel of the reflective spatial light modulator rotates through 90.degree. the direction of polarization of the light falling on (and reflected by) by the pixel. Thus, the polarized light reflected by the pixels of the reflective spatial light modulator either passes through a polarization analyzer or is absorbed by the polarization analyzer, depending on the direction of the electric field applied to each pixel.
The resulting optical characteristics of each pixel of both the transmissive and reflective spatial light modulators are binary: each pixel either transmits light (its 1 state) or absorbs light (its 0 state), and therefore appears light or dark, depending on the direction of the electric field.
To produce the grey scale required for conventional display devices, the apparent brightness of each pixel is varied by temporally modulating the light transmitted by each pixel. The light is modulated by defining a basic time period that will be called the illumination period of the spatial light modulator. The pixel electrode is driven by a drive signal that switches the pixel from its 1 state to its 0 state. The duration of the 1 state relative to the duration of the illumination period determines the apparent brightness of the pixel.
Ferroelectric liquid crystal-based spatial light modulators (both transmissive and reflective) suffer the disadvantage that, after each time the drive signal has been applied to a pixel electrode to cause the pixel to modulate the light passing through it, the DC balance of the pixel must be restored or a condition called "pixel sticking" will eventually result. Pixel sticking is a condition where a pixel will not change states despite a change in the direction of an electric field applied between a pixel electrode and the other electrode.
When operated in the DC balanced mode, pixel sticking is not problematic. This is done by defining a second basic time period called the balance period, equal in duration to the illumination period, and driving the pixel electrode with a complementary drive signal having 1 state and 0 state durations that are complementary to the 1 state and 0 state durations of the drive signal during the illumination period. The illumination period and the balance period collectively constitute a display period. To prevent the complementary drive signal from causing the display device to display a substantially uniform, grey image, the light source illuminating the light valve is modulated so that the light valve is only illuminated during the illumination period, and is not illuminated during the balance period. However, modulating the light source as just described reduces the light throughput of the light valve to about half of that which could be achieved if DC balance restoration were unnecessary. This means that a light source of approximately twice the intensity, with a corresponding increase in cost, is necessary to achieve a given display brightness. Additionally or alternatively, projection optics with a greater aperture, also with a corresponding increase in cost, are necessary to achieve a given brightness.
FIG. 1A shows part of a display device incorporating a conventional reflective light valve 10 that includes the reflective spatial light modulator 12. Other principal components of the light valve are the polarizer 14,the beam splitter 16 and the analyzer 18. The light valve is illuminated with light from the light source 20, the efficiency of which may be improved using a reflector 22 and collector optics 24 that concentrate the light towards the polarizer 14. The light output by the light valve passes to the output optics 26 that focus the light to form an image (not shown). The light valve, light source (including reflector and collector optics) and output optics may be incorporated into various types of display device, including miniature, wearable devices, cathode-ray tube replacements, and projection displays.
Light generated by the light source 20 passes through the polarizer 14. The polarizer polarizes the light output from the light source. The beam splitter 16 reflects a fraction of the polarized light output from the polarizer towards the spatial light modulator 12. The spatial light modulator is divided into a two-dimensional array of picture elements (pixels) that define the spatial resolution of the light valve. The beam splitter transmits a fraction of the light reflected by the spatial light modulator to the analyzer 18.
The direction of an electric field in each pixel of the spatial light modulator 12 determines whether or not the direction of polarization of the light reflected by the pixel is rotated by 90.degree. relative to the direction of polarization of the incident light. The light reflected by each pixel of the spatial light modulator passes through the beam splitter 16 and the analyzer 18 and is output from the light valve depending on whether or not its direction of polarization was rotated by the spatial light modulator. The light output from the light valve 10 passes to the output optics 26.
The light source 20 may be composed of LEDs. The LEDs are of three different colors in a color display. Other light-emitting devices whose output can be rapidly modulated may alternatively be used as the light source 20. As a further alternative, a white light source and a light modulator (not shown) may be used. The light modulator modulates the amplitude of the light generated by the light source to define the illumination period and balance period of the spatial light modulator. In a light valve for use in a color display device, the light modulator additionally modulates the color of the light output from the light source.
The polarizer 14 polarizes the light generated by the light source 20. The polarization is preferably linear polarization. The beam splitter 16 reflects the polarized light output from the polarizer towards the spatial light modulator 12, and transmits to the analyzer 18 the polarized light reflected by the spatial light modulator. The direction of maximum transmission of the analyzer is orthogonal to that of the polarizer in this example.
The spatial light modulator 12 is composed of the transparent electrode 28 deposited on the surface of the transparent cover 30, the reflective electrode 32 located on the surface of the semiconductor substrate 34, and the ferroelectric liquid crystal layer 36 sandwiched between the transparent electrode and the reflective electrode. The reflective electrode is divided into a two-dimensional array of pixel electrodes that define the pixels of the spatial light modulator and of the light valve. A substantially reduced number of pixel electrodes are shown to simplify the drawing. For example, in a light valve for use in a large-screen computer monitor, the reflective electrode could be divided into a two-dimensional array of 1600.times.1200 pixel electrodes. An exemplary pixel electrode is shown at 38. Each pixel electrode reflects the portion of the incident polarized light that falls on it towards the beam splitter 16.
A drive circuit (not shown), which may be located in the semiconductor substrate 34, applies a drive signal to the pixel electrode of each pixel of the spatial light modulator 12. The drive signal has two different voltage levels, and the transparent electrode 28 is maintained at a fixed potential mid-way between the voltage levels of the drive signal. The potential difference between the pixel electrode and the transparent electrode establishes an electric field across the part of the liquid crystal layer 36 between the pixel and transparent electrodes. The direction of the electric field determines whether the liquid crystal layer rotates the direction of polarization of the light reflected by the pixel electrode, or leaves the direction of polarization unchanged.
The reflective spatial light modulator 12 is structured as a quarter-wave plate in contrast to a transmissive spatial light modulator, which is structured as a half-wave plate. This difference arises because light passes through the reflective spatial light modulator twice, once before and once after reflection by the reflective pixel electrodes. The thickness of the layer of ferroelectric liquid crystal material in the liquid crystal layer 36 is chosen to provide an optical phase shift of 90.degree. between light polarized parallel to the director of the liquid crystal material and light polarized perpendicular to the director. The liquid crystal material is preferably a Smectic C* surface stabilized ferroelectric liquid crystal material having an angle of 22.5.degree. between its director and the normal to its smectic layers. Reversing the direction of the electric field applied to such a liquid crystal material switches the director of the material through an angle of about 45.degree.. Consequently, if the director is aligned parallel to the direction of maximum transmission of the analyzer 18 with one polarity of the electric field, reversing the direction of the electric field will rotate the direction of polarization of light reflected by the pixel through 90.degree.. This will align the direction of polarization of the light perpendicular to the direction of maximum transmission of the analyzer, and will change the pixel from its 1 state, in which the pixel appears bright, to its 0 state, in which the pixel appears dark.
In a miniature, wearable display, the output optics 26 are composed of an eyepiece that receives the light reflected by the reflective electrode 32 and forms a virtual image at a predetermined distance in front of the user (not shown). In a cathode-ray tube replacement or in a projection display, the output optics are composed of projection optics that focus an image of the reflective electrode on a transmissive or reflective screen (not shown). Optical arrangements suitable for use as an eyepiece or projection optics are well known in the art and will not be described here.
Since the direction of maximum transmission of the analyzer 18 is orthogonal to the direction of polarization defined by the polarizer 14, light whose direction of polarization has been rotated through 90.degree. by a pixel of the spatial light modulator 12 will pass through the analyzer and be output from the light valve 10 whereas light whose direction of polarization has not been rotated will not pass through the analyzer. The analyzer only transmits to the output optics 26 light whose direction of polarization has been rotated by pixels of the spatial light modulator. The pixels of the spatial light modulator will appear bright or dark depending on the direction of the electric field applied to each pixel. When a pixel appears bright, it will be said to be in its 1 state, and when the pixel appears dark, it will be said to be in its 0 state.
The direction of maximum transmission of the analyzer 18 can alternatively be arranged parallel to that of the polarizer 14,and a non-polarizing beam splitter can be used as the beam splitter 16. In this case, the spatial light modulator 12 operates in the opposite sense to that just described.
To produce the grey scale required by a display device notwithstanding the binary optical characteristics of the pixels of the light valve 10, the apparent brightness of each pixel is varied by temporally modulating the light reflected by the pixel, as described above. The drive circuit (not shown) for each pixel of the spatial light modulator determines the duration of the 1 state of the pixel in response to a portion of the input video signal 40 corresponding to the location of the pixel in the spatial light modulator.
FIGS. 1B-1G illustrate the operation of the exemplary pixel 38 of the conventional light valve 10 shown in FIG. 1A operating in a DC-balanced mode during three consecutive display periods. The remaining pixels operate similarly. In one embodiment of a conventional light valve, each display period corresponded to one frame of the input video signal 40. In another embodiment, each display period corresponded to a fraction of one frame of the input video signal. Each display period is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 1B.
FIG. 1C shows the bias voltage level applied to the transparent electrode 28. In this case, the bias voltage level is kept at a constant level of V/2, so that the changing the voltage applied to the exemplary pixel electrode 38 (as shown in FIG. 1D) from 0 to V reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 36. The level of the drive signal is V for a first temporal portion 1TP of each illumination period. The level of the drive signal is 0 for the second temporal portion 2TP constituting the remainder of the illumination period, and also for the first temporal portion 1TP of the subsequent balance period. The first temporal portion of the balance period has a duration equal to the first temporal portion of the illumination period. However, the level of the drive signal is 0 during the first temporal portion of the balance period, whereas the level of the drive signal is V during the first temporal portion of the illumination period. Finally, the level of the drive signal changes to V for the second temporal portion 2TP constituting the remainder of the balance period. Consequently, during the balance period, the level of the drive signal is 0 and V for times equal to the times that it was at V and 0, respectively, during the illumination period. As a result, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the duration of the first temporal portion 1TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of the input video signal 40.
FIG. 1E shows the effect of the spatial light modulator 12 on the direction of polarization of the light impinging on the analyzer 18. The direction of polarization is indicated by the absolute value of the angle a between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle a close to zero and absorbs light having an angle .alpha. close to 90.degree.. In each display period, the angle .alpha. has values corresponding to the pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 1F shows the modulation of the light source 20. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period.
FIG. 1G shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 38. Light is output from the pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the balance period of the display period because the light source 20 is OFF during the balance period. The light source being OFF for half the display period substantially reduces the perceived maximum brightness of the pixel, and of any image generated by a display device incorporating such a conventional light valve.
FIGS. 1H-1M illustrates the operation of the exemplary pixel 38 of the conventional light valve 10 shown in FIG. 1A operating in a known alternative DC-balanced mode during three consecutive display periods. The remaining pixels operate similarly. As with the previously described operation, each display period is composed of an illumination period (ILLUM) and a balance period (BALANCE) having equal durations, as shown in FIG. 1H. Each illumination and balance period, however, is further subdivided into a number of grayscale segments. Each grayscale segment represents the value of a digit in a binary number of a predetermined length. In the example shown, the length of the binary number chosen was four digits and each illumination and balance period was therefore broken into four segments. The first segment G1 is the shortest and represents the value of the binary number 0001. The duration of segment G1 is a fraction of each illumination and balance period determined by the formula: ##EQU1## where d is the position of the digit from the right and n is the total number of digits.
Thus, in our example the duration of the first segment G1 is determined by the value of the first digit and its duration is 1/16 of the illumination or display period. The second segment G2 is determined by the value of the second digit and its duration is 1/8 of the illumination or display period. The third segment G4 is determined by the third digit and its duration is 1/4 of the illumination or display period. Finally, the fourth segment G8 is determined by the value of the fourth digit and its duration is 1/2 of the illumination or display period.
FIG. 1I shows the bias voltage level applied to the transparent electrode 28. In this case, the bias voltage level is kept at a constant level of V/2, so that the changing the voltage applied to the exemplary pixel electrode 38 (as shown in FIG. 1J) from 0 to V reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 36.
The level of the drive signal is shown in FIG. 1J. In this case the level of grayscale from black to white desired for the output of a particular display period determines which of the grayscale segments have a drive signal with a voltage level of V and which grayscale segments have a drive signal with a voltage of 0. The number of possible gray variations between black and white is dependent on the number of grayscale segments. The are 2.sup.n possible gray variations where n is the number of grayscale segments. Thus, with the four grayscale segments shown, there are 16 possible gray variations between black and white. Using binary numerals to represent the 16 possible gray variations, black is represented by 0000 and white would be represented by 1111 with 1010 and 0101 being two possible gray variations representing a gray that is 10/16th white and a gray that is 5/16th white, respectively. The particular level needed is determined from the voltage level of the video signal 40.
Each digit in the binary number also represents the voltage level of the drive signal for the associated grayscale segment, with a 1 representing a drive signal voltage level of V in the associated grayscale segment in the illumination period and a drive signal voltage level of 0 in the associated grayscale segment in the following balance period. A binary 0 represents a drive signal voltage level of 0 in the associated grayscale segment in the illumination period and a drive signal voltage level of V in the associated grayscale segment in the following balance period. representing a voltage level of 0. As a result, the electric field applied to the liquid crystal material of the pixel averages to zero over the display period.
In the example shown, the drive signal for the illumination period of frame 1 has a voltage level of V in grayscale segments G8 and G2 and a voltage level of 0 in grayscale segments G4 and G1, representing the binary numeral 1010. This indicates a grayscale level that is 10/16 white. The drive signal for the illumination period of frame 2 has a voltage level of V only in grayscale segments G2, representing the binary numeral 0010. This indicates a grayscale level that is 2/16 white. The drive signal for the illumination period of frame 3 has a voltage level of V in grayscale segments G8, G4, and G2, representing the binary numeral 1110. This indicates a grayscale level that is 14/16 white.
FIG. 1K shows the effect of the spatial light modulator 12 on the direction of polarization of the light impinging on the analyzer 18. The direction of polarization is indicated by the absolute value of the angle .alpha. between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle a close to zero and absorbs light having an angle .alpha. close to 90.degree.. In each display period, the angle .alpha. has values corresponding to the pixel being bright and dark for equal times due to the need to restore the DC balance of the pixel.
FIG. 1L shows the modulation of the light source 20. The light source is ON throughout the illumination period of each display period, and is OFF during the following balance period.
FIG. 1M shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 38. Light is output from the pixel only during the grayscale segments of illumination period of each display period when the drive signal voltage is V. No light is output during the portions of the illumination period during grayscale segments with a drive signal of 0 volts. Moreover, no light is output during the balance period of the display period because the light source 20 is OFF during the balance period. The light source being OFF for half the display period substantially reduces the perceived maximum brightness of the pixel, and of any image generated by a display device incorporating such a conventional light valve.
The need to maintain the DC balance of the ferroelectric liquid crystal material of the spatial light modulator 12 means that the reflective light valve just described suffers from a similar light throughput problem to the transmissive light valves described above.
One alternative to maintaining the DC balance of the ferroelectric liquid crystal material of the spatial light modulator is to operate the spatial light modulator in a non-DC balanced mode and to "spike" the voltage applied to the transparent electrode 28. Spiking the voltage means that a relatively large positive voltage, as compared to the normal V/2 bias voltage, is applied to the transparent electrode followed immediately by a relatively large negative voltage. Alternatively, the relatively large negative voltage may be applied prior to the relatively large positive voltage. Typical voltage levels applied to the transparent electrode would be V/2=2.5 volts with a spiking voltage of .+-.6 volts.
FIGS. 1N-1S illustrate the operation of the exemplary pixel 38 of the conventional light valve 10 shown in FIG. 1A operating in a non-DC-balanced mode during three consecutive display periods. The remaining pixels operate similarly. In one embodiment of a conventional light valve, each display period corresponded to one frame of the input video signal 40. In another embodiment, each display period corresponded to a fraction of one frame of the input video signal. Each display period is composed of an illumination period (ILLUMINATION) and a spiking period (SPIKE) having a substantially shorter duration than the illumination period, as shown in FIG. 1N. The duration of illumination period may be four or more times the duration of the spiking period.
FIG. 1O shows the voltage level applied to the transparent electrode 28. During the illumination period, the voltage applied is kept at a constant level of V/2. During the spiking period, however, the voltage applied is rapidly raised to a positive spiking voltage V.sub.1. Roughly halfway through the spiking period, the voltage applied to the transparent electrode is rapidly dropped to a negative spiking voltage -V.sub.1 before returning to the V/2 level for the next illumination period.
FIG. 1P shows the voltage applied to the exemplary pixel electrode 38. During the illumination period, changing the applied voltage from 0 to V reverses the direction of the electric field applied to the ferroelectric liquid crystal layer 36. The level of the drive signal is V for a first temporal portion 1TP of each illumination period. The level of the drive signal is 0 for the second temporal portion 2TP constituting the remainder of the illumination period, and also for the first half of the spiking period. Keeping the voltage level of the exemplary pixel at zero volts while the level of the voltage applied to the transparent electrode is at spiking voltage V.sub.1 maximizes the electric field applied across the ferroelectric liquid crystal material 36, postponing the onset of pixel sticking. Finally, the level of the drive signal changes to V for the second half of the spiking period, corresponding to the application of the negative spiking voltage -V.sub.1 to the transparent electrode. This maximizes the reverse electric field applied across the ferroelectric liquid crystal material 36, postponing the onset of pixel sticking.
In the example shown, the duration of the first temporal portion 1TP of the drive signal is different in each of the three illumination periods. The duration of the first temporal portion, and, hence, of the second temporal portion, of each illumination period depends on the voltage level of the corresponding sample of the input video signal 40.
FIG. 1Q shows the effect of the spatial light modulator 12 on the direction of polarization of the light impinging on the analyzer 18. The direction of polarization is indicated by the absolute value of the angle .alpha. between direction of polarization of the light impinging on the analyzer and the direction of maximum transmissivity of the analyzer. The analyzer transmits light having an angle .alpha. close to zero and absorbs light having an angle .alpha. close to 90.degree.. In each display period, the angle .alpha. has values corresponding to the pixel being bright and dark for unequal times and operation is said to be in a non-DC-balance mode.
FIG. 1R shows the modulation of the light source 20. The light source is ON throughout the illumination period of each display period, and is OFF during the following spiking period.
FIG. 1S shows the light output from the exemplary pixel of the light valve 10 controlled by the pixel electrode 38. Light is output from the pixel only during the first temporal portion of the illumination period of each display period. No light is output during the second temporal portion of the illumination period. Moreover, no light is output during the spiking period of the display period because the light source 20 is OFF during the spiking period. The light source being ON for substantially more than half the display period substantially improves the perceived maximum brightness of the pixel for a given light source 20 over the same light valve operating in a DC balanced mode. Similarly, the image generated by a display device incorporating a light valve operating in a non-DC-balanced mode would be substantially brighter (nearly twice as bright) as the same display device incorporating a light valve operating in a DC-balanced mode.
An alternative non-DC-balanced mode with spiking would be to utilize the previously described segmented grayscale operation (not shown) in place of the two temporal periods 1 TP and 2 TP shown in FIG. 1P.
A light valve operating in a non-DC-balanced mode with spiking certainly improves the performance of the light valve over similar light valves working in a non-DC-balanced mode without sticking by substantially delaying the onset of pixel sticking. FIG. 2A is a graph showing the onset of pixel sticking over time in a light valve operating in a non-DC-balanced mode without spiking. Substantial pixel sticking can be observed in the light valve within a half hour of operation.
FIG. 2B is a graph showing the onset of pixel sticking over time in a similar light valve operating in a non-DC-balanced mode with spiking. The onset os pixel sticking is delayed and does not become a significant problem until after nearly 5 hours of operation.
While spiking provides a delay in on the onset of pixel sticking, the onset of pixel sticking within hours still makes non-DC-balanced operation of a ferroelectric liquid crystal-based spatial light modulator unpractical for most display applications. Consequently, what is needed is a ferroelectric liquid crystal-based light valve that can operate in a non-DC-balanced mode for extended periods without pixel sticking.